The present invention relates to a method for solidifying high and medium radioactivity and/or actinide containing aqueous waste concentrates or fine-grained solid wastes suspended in water for final noncontaminating storage in which the waste concentrates or the suspensions are formed into pellets or granules together with absorbing and/or hydraulically binding inorganic material and thereafter subjecting the resulting pellets or granules to a ceramic firing process in order to produce solid sintered bodies.
It has been known for a long time to solidify radio-active aqueous solutions by first reducing the volume of such wastes, thereby concentrating the radioactive substances, and thereafter treating the concentrates either by (1) subjecting them together with glass formers to a heat treatment until the radioactive substances become distributed throughout the resulting glass melt and then having the melt solidify into a solid body, or (2) by mixing the concentrated wastes with silicate-containing clays or with ion exchangers, respectively, and firing the resulting mix ceramically so as to form a solid body.
Some of the drawbacks of producing glass blocks having radioactive waste substances incorporated therein include the need to use relatively complicated and expensive apparatus which must be operated by trained personnel. Moreover, in the course of prolonged storage, decomposition of the glass structure may occur due to the continued emission of radiation and heat energy by the incorporated highly radioactive substances with the result that the resistance of the glass structure to leaching deteriorates with time and its ability to effectively retain radioactive materials is diminished, especially as compared to the relatively good leaching properties of nondecomposed glass waste blocks.
When clay-radionuclide mixtures are fired according to the prior art, the quality of the solidified products containing high concentrations of radioactive substances has not been sufficient for final storage purposes.
An additional problem encountered with prior art solidification by glass and fired clay processes is that during the high temperature stages, significant quantities of radioactive substances evaporate from the not yet solidified waste. These escaping impurities must be trapped and removed by complicated waste gas purification techniques involving solids filters, gas washing columns and condensate separators.
German Pat. No. 1,127,508 to Alberti proposes mixing aqueous atomic waste with fireproof cement and then increasing the density of the resulting hardened block by ceramic firing to produce a sintered body which is resistant to leaching. In order to increase the mechanical stability of the hardened block, the patent suggests adding fireproof additives such as fire clay or brick chips to the fireproof cement. For example, a cylindrical molded body was produced from molten alumina cement and radioactive liquid. The molded body, after hardening, was uniformly heated for a period of 5 hours to a temperature of 500.degree. C. to evaporate excess water. The molded body was then rapidly brought to a firing temperature of, for example, 1100.degree. C. and kept at this temperature for about 2 to 4 hours after which the molded body was cooled slowly. No information is given in the patent about the radioactivity of the radioactive liquid being treated. There is also no disclosure in the patent as to the quantities of liquid being treated in the 3-liter vessels used by Alberti or as to the water-cement values, or the like. Results of leaching experiments likewise were not disclosed.
The Alberti process may be useful for the solidification of low radioactivity aqueous wastes, but it is very expensive and unnecessarily complicated. Further, it cannot be used for the solidification of high or medium radioactivity and/or actinide containing aqueous wastes.
Medium activity waste solutions have been solidified in cement, concrete or bitumen at temperatures of more than about 150.degree. C. Solidification of medium activity waste solutions with cement, concretes or bitumen leads to end products which have low thermal stability and relatively low radiation resistance over extended periods of time. As a result, special safety measures become necessary when depositing these products for intermediate or final storage.
When seeking to store actinide concentrates, the intensive development of radiolysis gases and heat in the product, as a result of the radioactive decomposition of the actinides, renders bitumen, cement or concrete solidification processes completely unsuitable.
Suspended combustion ashes or ion exchangers have previously been solidified in cement and put into barrels which act as sheaths. The thus sheathed, solidified products have then been put directly into storage. The properties of such blocks, however, particularly with respect to mechanical stability and leaching resistance, are not particularly good, so that this type of solidification is used only for weakly active wastes.
In an effort to overcome the aforementioned problems of the prior art, a process for solidifying high and medium radiioactivity and/or actinide containing aqueous waste concentrates or fine-grained solid wastes suspended in water for final noncontaminating storage was disclosed in U.S. patent application Ser. No. 914,152 filed June 9, 1978, assigned to the same assignee as the present application and herein incorporated by reference. According to that application, the waste concentrates or suspensions are initially set, by evaporation, to a water content in the range between 40 and 80 percent by weight, and a solid content whose metal ion and/or metal oxide component lies between 10 and 30 percent by weight of the evaporate or concentrate being formed and a pH between 5 and 10. The evaporate is kneaded with a clay-like substance containing a small quantity of cement or such a clay-like substance or mixture of a clay-like substance with a small quantity of cement containing an additive for suppressing the volatility of the alkalis or alkali earths and/or an additive for suppressing the volatility of any decomposing anions which may be present in the evaporate or concentrate from the group including sulfate, phosphate, molybdate and uranate ions. Molded bodies are produced from the resulting kneaded mass with the aid of molds into which the mass is pressed or with an extrusion press which must be followed by a cutting device. The molded bodies are then heat treated and subsequently fired at high temperatures to form practically undissolvable mineral phases. The molded bodies of fired mineral phases are enclosed on all sides in a dense, continuous ceramic or metallic matrix.
The latter process necessitates the use of so-called hot cells for the solidification of the aqueous wastes due to their radioactivity. In addition, the use of molds or extrusion presses under such conditions is complicated and requires a lot of space. A need therefore exists for a simple and efficient process wherein it is possible to avoid the aforementioned disadvantages wherein the desired pellets can be formed, without difficulty, in an entirely continuous manner.